Telechelic and block copolymers are well known in the art and are employed for a variety of purposes. Of the block copolymers, those having the configuration AB(BA).sub.n wherein n has a value of from about 1-10, particularly 1, that is block copolymers having a simple ABA, configuration are highly important items of commerce for use in adhesives and as elastomers. The A block is preferably an olefin polymer, especially a styrene homopolymer or copolymer. Usually the B block is an elastomeric polymer, especially a polymer of a diene such as a polymer of butadiene or isoprene. To obtain block copolymers of maximum uniformity, it is usually desirable to initiate polymerization employing a multifunctional lithium compound. In the case of an ABA block copolymer (i.e. a triblock copolymer), a difunctional compound would be employed. In the case of an AB(BA).sub.3 block copolymer, a tetrafunctional initiator would be utilized, etc.
Anionic polymerization is well known in the art, for example U.S. Pat. Nos. 4,431,777 and 4,427,837 disclose suitable anionic polymerization processes. Multifunctional initiators are well known and have been previously used in polymerizations. Such initiators and their use are shown in the following U.S. Pat. Nos.: 4,169,115; 4,172,100; 4,172,190; 4,427,837; 4,196,154; and 4,205,016. The teachings of the preceding patents are incorporated herein by reference thereto.
Particularly desirable multifunctional lithium containing compounds are selected from the group consisting of the formula: ##STR1## wherein R.sub.1 is independently each occurrence hydrogen or an inert radical having from 0 to 16 carbon atoms;
R.sub.2 is a divalent organic radical having at least 6 carbon atoms, R.sub.2 having at least one aromatic ring and the aromatic ring being directly attached to a carbon which is attached to an aromatic ring of the above formula. PA1 R.sub.3 is independently each occurrence selected from the group consisting of alkyl, cycloalkyl, aromatic, mixed alkyl/aromatic, and mixed cycloalkyl/aromatic radicals containing from 1 to 20 carbon atoms. Especially preferred are initiating compounds of the formula: ##STR2## wherein R.sub.1 and R.sub.3 are as previously defined. PA1 R" independently each occurrence is a C.sub.1-20 alkyl group or an inertly substituted derivative thereof; and PA1 n is zero or one.
By the term "inert" as used in this context is meant substitutents that do not interfere with the desired anionic polymerization. In a most preferred embodiment, R.sub.1 is selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, aryl and mixtures thereof. Specific examples of difunctional initiators (DFIs) corresponding to the above formula are 1,3-phenylene bis(3-methyl-1-phenylpentylidene)bis-(lithium), 1,3-phenylene bis(3-methyl-1-(4-methylphenyl)-pentylidene) bis(lithium), 1,3-phenylene bis(3-methyl-1-(4-ethylphenyl)-pentylidene) bis(lithium), 1,3- phenylene bis(3-methyl-1-(4-(1,1-dimethylethyl)phenyl)pentylidene) bis(lithium), and 1,4-phenylene bis(3-methyl-1-(4-dodecylphenyl)-pentylidene) bis(lithium).
Generally, the above described multifunctional lithium containing initiators result in the preparation of polymerization products having a broader molecular weight range than is often desired. In certain applications, particularly adhesives, this may result in inferior product properties. As a particular example we have now found that styrene-isoprene-styrene triblock polymers of narrower molecular weight distribution tend to produce adhesive formulations having improved shear hold strength and the triblock polymer itself normally possesses improved tensile rupture strength compared to broader molecular weight versions thereof. Accordingly, for adhesive applications it would be desirable to prepare block copolymers having a relatively narrow molecular weight distribution, especially distributions wherein the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) is less than or equal to 1.2 and preferably less than 1.1.
A technique that may be employed to produce narrow molecular weight distribution polymers is the addition of certain polar compounds to the reaction mixture. Disadvantageously, however, in the polymerization of isoprene containing mixtures, the resulting polymer product possesses a high percentage of undesired vinyl functionality producing 1,2- or 3,4-addition polymer products. Such addition products are collectively referred to hereinafter as polymers having 3,4- addition microstructure. Polymers having this microstructure likewise generally possess inferior elastomer properties. Consequently, there remains a need in the art to provide a technique for the anionic polymerization of monomers to yield narrow molecular weight distribution polymers, especially block copolymers, having physical properties and molecular structure that are not otherwise disadvantageously affected.
It would be desirable if there were available an improved process for the preparation of block copolymers of the AB(BA).sub.n variety.
It would also be desirable if there were available an improved process for the preparation of block copolymers of the AB(BA).sub.n variety which would result in products having a narrow molecular weight distribution.
It would also be desirable if there were available an initiator system which would provide such polymers having a narrow molecular weight distribution.